Vim search replace regex + incremental function

I'm currently stuck in vim trying to find a search/replace oneliner to replace a number with another + increment for each new iteration = when it finds a new match.

I'm working in xml svg code to batch process files Inkscape cannot process the text (plain svg multiline text bug).

<tspan
       x="938.91315"
       y="783.20563"
       id="tspan13017"
       style="font-weight:bold">Text1:</tspan><tspan
       x="938.91315"
       y="833.20563"
       id="tspan13019">Text2</tspan><tspan
       x="938.91315"
       y="883.20563"
       id="tspan13021">✗Text3</tspan>

etc.

So what I want to do is to change that to this result:

<tspan
       x="938.91315"
       y="200"
       id="tspan13017"
       style="font-weight:bold">Text1:</tspan><tspan
       x="938.91315"
       y="240"
       id="tspan13019">Text2</tspan><tspan
       x="938.91315"
       y="280"
       id="tspan13021">✗Text3</tspan>

etc.

So I duckducked and found the best vim tips resource from zzapper, but I cannot understand it:

convert yy to 10,11,12 :

:let i=10 | ’a,’bg/Abc/s/yy/\=i/ |let i=i+1

I then adapted it to something I can understand and should work in my home vim:

:let i=300 | 327,$ smagic ! y=\"[0-9]\+.[0-9]\+\" ! \=i ! g | let i=i+50

But somehow it doesn't loop, all I get is that:

<tspan
       x="938.91315"
       300
       id="tspan13017"
       style="font-weight:bold">Text1:</tspan><tspan
       x="938.91315"
       300
       id="tspan13019">Text2</tspan><tspan
       x="938.91315"
       300
       id="tspan13021">✗Text3</tspan>

So here I'm seriously stuck. I cannot figure out what doesn't work :

My adaptation of the original formula ?
My data layout ?
My .vimrc ?

I'll try to find other resources by myself, but on that kind of trick they are pretty rare I find, and like in zzapper tips, not always delivered with a manual.

Solution

One way to fix it:

:let i = 300 | g/\m\<y=/ s/\my="\zs\d\+.\d\+\ze"/\=i/ | let i += 50

Translation:

let i = 300 - hopefully obvious
g/\m\<y=/ ... - for all lines matching \m\<y=, apply the following command; the "following command" is s/.../.../ | let ...; the regexp:
- \m - "magic" regexp
- \< - match only at word boundary
s/\my="\zs\d\+.\d\+\ze"/\=i/ - substitute; the regexp:
- \m - "magic" regexp
- \d\+ - one or more digits
- \zs...\ze - replace only what is matched between these points
- \=i - replace with the value of expression i
let i += 50 - hopefully obvious again.

For more information: :help :g, :help \zs, :help \ze, help s/\\=.